Polyolefin resins, which are prepared from aliphatic olefins, are known in the art as a class of thermoplastics characterized by excellent physical properties. For example, polyolefin resins demonstrate a high resistance to stress cracking, a high tensile strength and a high stability under load. These physical properties make this class of thermoplastics particularly useful in the manufacture of pipe, film, wire, coated molded objects, etc.
However, polyolefins demonstrate a high amount of degradation at the high temperatures typically required in the processing of the polymers into useful finished articles. This drawback associated with the polyolefin resins is discussed, e.g., in U.S. Pat. No. 3,574,165.
Moreover, polyolefin resins are further detrimentally affected by oxidative degradation caused by the formation of peroxides in the polymer backbone. These peroxides decompose under the influence of heat and light initiating chain scission or crosslinking reactions in the polymer which ultimately result in the loss of physical properties. The compounds of this invention effectively decompose the peroxides before they can adversely affect the polymer.
In order to stabilize polyolefins against the deleterious effects of both thermal and oxidative degradation, it is known in the art to incorporate small amounts of various phenolic or arylamine antioxidants into the resin in combination with other antioxidants, such as sulfide containing antioxidants. For example, U.S. Pat. Nos. 3,180,850, 3,258,493, 3,293,209, 3,574,165, 3,652,680 and 3,772,246 all teach the use of a combination of these types of antioxidants as ingredients in resins to protect polyolefin resins from both thermal and oxidative degradation.
Conventional sulfide antioxidants, used to stabilize polyolefin resins, are the esters of thiodipropionic acid, particularly the dilauryl thiodipropionate and the distearyl thiodipropionate (DLTDP and DSTDP, respectively). Although the thiodipropionates are effective in stabilizing polyolefin resins, they suffer from the drawback of being hydrolytically unstable due to the presence of an ester functionality. Due to this hydrolytic instability, the thiodipropionates may be leached out of the polyolefin resins and articles made with these resins by hot water and in particular, hot water containing soap and detergents. The loss of the thiodipropionate stabilizer will cause premature deterioration, such as embrittlement and cracking of the polyolefins. This is particularly disadvantageous when the resins are used in hot water applications such as in hot water pipes, tubing applications and as components for automatic laundry and dishwashing equipment, etc. The drawbacks of the thiodipropionates are discussed in U.S. Pat. Nos. 3,652,680 and 3,772,246.
The present invention provides organic sulfide antioxidants which may be used to stabilize polyolefin resin compositions, which not only provide excellent results as antioxidants and processing stabilizers, but also possess hydrolytic stability. Moreover, the antioxidant compounds of the present invention may be incorporated into the polyolefin resins to protect the polyolefins both during processing and after the resins have been formed into articles.